A supplement is requested to expand ongoing research into the developmental changes of lung elastin, in particular the diminishing ratio of microfibrillar to amorphous elastin. Observed transitions in amino acid composition will be correlated with morphological differences by electronmicroscopy. Newly available staining procedures will be used to visualize the amorphous as well as the microfibrillar component and establish the developmental stages at which maturity of lung elastin is reached. Since the microfibrillar component appears to be a glycoprotein the amino acid and peptide studies will be supplemented by carbohydrate analyses. Methods will also be adopted to monitor possible effect of agents such as corticosteroids which may accelerate maturation of the lung and thus decrease the incidence of RDS in high risk pregnancies.